The aim of this investigation was to observe the effect that collector slope has on the optical efficiency and corresponding efficiency curves for an evacuated tube collector.

During this study the irradiance from the solar simulator was always normal to the absorber surface as illustrated in Figure 5a. This was purely an attempt to monitor the effect of collector slope and not the relative characteristic dealing with incidence angle of irradiance for a fixed collector angle. The irradiance power density was 800 ± 50 Wm-2, the mass flow-rate was held at 0.07 kgs-1 throughout the experiments, the slope p was varied between 0° and 60° with respect to the horizontal plane.

The observations showed that the optical efficiency remained constant with collector slope as summarised in the table below Figure 5b, the average value of the optical efficiency was 0.83 ± 0.02 over the range. The a1 value of linearly dependent collector losses were found to decrease by ~9% whereas the less influential square dependences terms a2 where found increase by 750%, with increasing slope. The raw data efficiency curves were corrected to an irradiance of 800 Wm-2 and presented according to standard EN12975-2 as seen in Figures 5 c and d. The overall result showed that increasing the slope was found to have a beneficial effect on collector performance. This improvement was assumed to be due to the fact that an increased slope will promote thermosyphonic mechanisms within the collector tube.

Further investigation at a collector slope of 90° was not carried out due to a technical problem with the solar simulator, this data set will be completed at a later date.

Figure 5 [a] Schematic of the experimental set-up, [b] summary table of the optical efficiencies and corresponding loss coefficients for different slopes, [c] plot of the raw data efficiency curves at 800Wm-2 and [d] plot of the standardised efficiency curves at 800Wm-2